An observation technique of propagating SCC's crack tip for type 304 stainless steel

The purpose of this work is to understand the microstructure of the stress corrosion crack tip formed on sensitized type 304 stainless steel in a sodium thiosulfate solution at room temperature. The stress corrosion crack tips of bulk specimens were observed by the transmission electron microscope and the slow strain rage test (SSRT) were conducted in the solution under controlled current conditions. Additionaly, x-ray diffraction measurement were conducted. Main results were as followes; (1) Strain-induced martensites (ε and α') were observed near some SCC crack tips on the specimen. (2) It is considered that strain-induced martensites have no influence on the SCC in this corrosion system. (3) X-ray diffraction measurement for the fracture surface showed that the austenitic structure were increased as the ratio of IGSCC fracture surface. (4) Results obtained from SSRT under controlled current conditions and above results showed that active path corrosion (APC) was most relevent process on crack propagation in this corrosion system. (author)